There are many ways of extracting water from air but many of them are struggling with efficiency and that they demand a lot of energy.
Heat pumps are efficient when it comes to transferring heat. The ratio between the energy that the pump may transfer and the total amount of energy that may be transferred is referred to Coefficient of Performance (COP). The COP factor depends on a number of parameters such as temperature and what kind of cooling media is used. When the heat pump transfers heat from one place to another a cold and a warm surface is formed in the heat pump. The warm surface may be used for heating while the cold surface may be used for cooling.
The cold surface may also be used to condense vaporised water from the air. The amount of water that may be condensed from 1 m3 air depends on its initial temperature and the amount of vapour in the air as well as to what temperature the air is cooled.
This method of condensing vapour from air has a disadvantage since the condensed water freezes at 0° C. This results in the formation of ice on the cold side and that the condensed water may not be conveyed. Since the air has a dewpoint below 0° C. the air needs to be cooled below 0° C. in order for the vapour to condense. This limits of course when a heat pump may be used to condense vapour from air. Today one way of solving the problem is to regularly stop the heat pump and heat the cold surface and remove the formed ice. However this is both time and energy consuming.
Another method used for extracting vapour is to adsorb water using a hygroscopic material. By using a hygroscopic material the vapour in the air may be adsorbed and stored in the material. The amount of water that may be stored in a hygroscopic material depends on the materials ability to adsorb at different humidity. When the hygroscopic material is heated the vapour pressure of the material increases. When the vapour pressure in the hygroscopic material becomes higher than the surrounding vapour pressure the adsorbed water is vaporised. In this way hygroscopic materials may be regenerated and reused. The amount of water that may be adsorbed/desorbed per second is described by:{dot over (r)}=kbAσ(Pm−Pa)  (1)where {dot over (r)} is the amount of water adsorbed/desorbed per second, k is a material constant, A is the area of the hygroscopic material and Pm is the vapour pressure in the hygroscopic material and Pa is the vapour pressure in the air.
In order for the adsorbed water to leave the hygroscopic material it needs to be vaporized and for that vaporisation energy is needed. This energy is taken from the heat and creates cooling. This cools the hygroscopic material resulting in a lower vapour pressure in the hygroscopic material when the water is vaporized. More heat needs then to be added in order to keep the material warm.
The systems of prior art suffers from some drawbacks such as formation of ice and cooling of hygroscopic materials due to vaporization. Overcoming these issues would lead to more efficient methods to control air humidity and to condense water vapour.